Description

Every time a consecutive group of duplicate elements appears in the
range [first, last), the algorithm unique removes all but the
first element. That is, unique returns an iterator
new_last such that the range [first, new_last) contains no
two consecutive elements that are duplicates. [1]
The iterators in the range [new_last, last) are all still
dereferenceable, but the elements that they point to are unspecified.
Unique is stable, meaning that the relative order of elements that
are not removed is unchanged.

The reason there are two different versions of unique is that there
are two different definitions of what it means for a consecutive group
of elements to be duplicates. In the first version, the test is
simple equality: the elements in a range [f, l) are duplicates if,
for every iterator i in the range, either i == f or else *i == *(i-1).
In the second, the test is an arbitrary Binary Predicatebinary_pred: the elements in [f, l) are duplicates if, for every
iterator i in the range, either i == f or else
binary_pred(*i, *(i-1)) is true. [2]

Definition

Defined in the standard header algorithm, and in the nonstandard
backward-compatibility header algo.h.

Notes

[1]
Note that the meaning of "removal" is somewhat subtle. Unique,
like remove, does not destroy any iterators and does not change
the distance between first and last. (There's no way that it
could do anything of the sort.) So, for example, if V is a
vector, remove(V.begin(), V.end(), 0) does not change
V.size(): V will contain just as many elements as it did before.
Unique returns an iterator that points to the end of the resulting
range after elements have been removed from it; it follows that the
elements after that iterator are of no interest. If you are operating
on a Sequence, you may wish to use the Sequence's erase
member function to discard those elements entirely.

[2]
Strictly speaking, the first version of unique is redundant:
you can achieve the same functionality by using an object of class
equal_to as the Binary Predicate argument. The first version
is provided strictly for the sake of convenience: testing for equality
is an important special case.

[3]BinaryPredicate is not required to be an equivalence
relation. You should be cautious, though, about using unique with a
Binary Predicate that is not an equivalence relation: you could
easily get unexpected results.